Hereditary spastic paraparesis (HSP) is characterized by progressive spasticity and weakness of the legs with or without additional abnormalities. Reports of psychiatric disorders in patients with HSP are limited to mood disturbances rather than to psychosis. We had noted significant psychotic illness in several patients recruited to a survey of HSP in Ireland and therefore set about examining the frequency and nature of psychosis in our patients with HSP. Cases with HSP and psychotic illness were identified from a nationwide epidemiological and clinical study. Psychiatric case notes were reviewed and Operational Diagnostic Criteria Checklist (OPCRIT) applied. Six patients from four families with HSP had evidence of psychosis in addition to paraparesis. OPCRIT diagnoses were 'narrow schizophrenia' (n = 2), 'broad schizophrenia' (n = 2) and 'schizo-affective/manic disorder' (n = 2). Patients were from families with Kjellin's syndrome and SPG4-HSP but not other kindreds and psychosis was not evident in family members without HSP. We found a higher than expected rate of psychosis in the Irish HSP population. Two groups of HSP patients may have increased risk of developing psychosis: those with Kjellin's syndrome and those with SPG4-HSP.